1. Field of the Invention
The present invention relates in general to path length measurement, and more particularly to coherent optical path length measurement systems capable of varying degrees of resolution.
2. Background Information
The invention described and claimed herein comprises a novel method and device for measurement of optical path length.
There are many optical techniques for the remote measurement of path lengths. They can be categorized into two main groups: direct optical detection and coherent optical detection. In a direct optical detection scheme, a laser pulse is emitted from a probing station toward a remote target from which a return pulse is reflected. The reflected return pulse is collected, and its arrival time relative to the reference time is determined. Because the distance that the pulse travels is equal to the product of the speed of light and the elapsed time, the distance of the remote target from the probing station can be determined simply by measuring the delay time of the reflected laser pulse. The spatial resolution of such direct optical detection schemes is, however, quite limited due to the lack of detection electronics capable of detecting very minute time delays and the very short time delays of the laser pulses.
In a coherent optical detection scheme, the phase coherence properties of a laser emission are exploited to offer information about the relative magnitudes between two path lengths. Interferometers are typically used in this approach, with one arm of the interferometer used as the reference path to gauge the length of the other arm, which contains the object under investigation. Laser emissions traveling through the two branches of the interferometer are combined onto a photosensor, such as a photodiode or a photoconductor, which produces a photocurrent proportional to the overall intensity of the interference pattern. These coherent optical detection techniques can resolve path length differences as small as a fraction of the wavelength of the laser light used.
However, several drawbacks are associated with this technique. First, the interferometer setup requires very stringent optical alignment in order to achieve sufficient mixing of the laser beams inside the conventional photosensor. Second, the technique""s resolution is limited; it is a fraction of the laser wavelength and typically on the order of 1 nanometer. Additionally, the photocurrent that is generated by these conventional photosensors is a sinusoidal function that varies with the relative path length difference between the reference path and the object path. As a result, the intensity of the generated photocurrent is the same when the path length difference differs by an integer multiple of the laser wavelength. This ambiguity makes it difficult to determine the path length difference when the reference path and the object path are of significantly different lengths; methods for dealing with this ambiguity require a priori knowledge of the approximate distance being measured, and selection of appropriate components.
The foregoing problems are overcome, and other advantages are provided by a system, in accordance with the invention, which comprises a laser source and/or dispersive elements which produces a laser emission, either continuous-wave or pulsed in nature, with chirped frequencies and a optical frequency photosensor capable of measuring optical frequency differences between the two interfering laser beams by generating photocurrents linearly proportional to the relative frequency difference between them.
It is an object of the invention to provide a method for measuring distances which is more accurate than conventional methods.
It is another object of the invention to provide a method for measuring distances without a priori knowledge of the approximate distance to be measured.
A principal feature of the invention is the use of chirped frequencies.
Among the advantages of the invention are increased accuracy and autoranging.
These and other objects, features and advantages will be apparent from the discussion which follows. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its advantages and objects, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.